DNA is like a blueprint for our cells. It expresses how cells should perform produce certain proteins and conduct certain functions to keep us healthy and acts as a kind of instruction booklet on how to replicate cells correctly when they reproduce. Basically, like the blueprint of life. So, as you can imagine the DNA of our cells is quite an import part about what makes us who we are and what keeps us working healthily.
While DNA does break down overtime due to our normal aging process, a number of external factors can increase damage to our DNA contributing to accelerated aging and other effects such as the potential to develop skin cancers.